1. Field of the Invention
The present invention relates to a leak detection system for a part having an interior cavity.
2. Description of the Prior Art
There are many previously known systems for detecting leaks in parts having an interior cavity. In many of these previously known systems, the part is pressurized with a preselected gas, such as helium, and placed within a closed chamber. The chamber has been evacuated and a gas detector coupled to the chamber detects the presence of the preselected gas within the evacuated test chamber. The presence of the preselected gas in the gas chamber after evacuation, of course, is indicative of a leak in the part under test.
A second known system has the vacuum inside the part and helium is introduced in small puffs about the exterior. Presence of helium in the evacuated interior indicates a leak in the part near where the helium was last introduced. This kind of testing apparatus cannot be calibrated for actual leak rate and size of hole due to uncertainty in the quantity of applied helium.
A third known system is called a "helium sniffer" detector. The pan is pressurized with helium and a small probe attached to the helium mass spectrometer is moved about the exterior. Wherever a hole exists between the exterior surface and interior cavity, the helium detector will register a positive signal. In general, for all possible test geometries, this kind of system cannot be given an absolute leak rate calibration.
These previously known leak detection systems, however, are not well suited for high production leak testing of the type required by EPA regulations for the automotive industry. Under such regulations, parts are deemed unacceptable if they leak the working fluid at rates in excess of some threshold value. Parts with leaks below this threshold are deemed acceptable. The aforementioned systems are meant to check for fully hermetic seals. Many of these previous systems cannot be calibrated in any way and are incapable of detecting a threshold leak rate greater than zero. Of the types that can be calibrated to absolute leak rate, the systems require a hard vacuum to assure consistent results. In particular, the disadvantages of such systems that can be calibrated are their complex vacuum pumping apparatus and their required time during production testing to evacuate the test chamber to operating vacuums. The previous detection systems are much more demanding than necessary. Those systems use fully evacuated chambers with high mechanical complexity and reduced reliability.